1. Field of the Invention
This invention relates to partial discharge detection devices. More particularly, it relates to a device capable of monitoring and detecting partial discharge in an insulation medium of an electrical system and controlling the electrical system coupled thereto in response to such detection.
2. Description of Prior Art
Partial discharge is an electrical phenomenon that can occur within an insulation medium in any electrical system having electrical conductors. Recently, the term partial discharge has been used to define a specific phenomenon that is different than that known as corona discharge. Partial discharge is a type of localized internal electrical discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. Corona, on the other hand, is an external electrical discharge occurring as a result from the ionization of gases of the surrounding air by the high voltage (that which exceeds the critical value). Corona is often heard as acoustical noise about high-voltage transmission lines, representing sustained discharges in gases that have been energized by an intense electric field near the electrical conductors. Corona can often been seen as a bluish purple glow on the surface of and adjacent a conductor. In other words, where partial discharge is an internal discharge, corona is an external discharge. When reviewing prior art which was published before the 1980's, it is common for authors to refer to corona when really they are addressing partial discharge. For the purposes of this disclosure, a reference using the word corona will be understood to be describing the internal electrical phenomenon occurring within an insulation system known as partial discharge as defined hereinabove.
Partial discharge occurring within an insulation medium can be destructive upon the insulator. In particular, the free electrons in the insulator, accelerated by the electric field, which thereby produces the ionization, collide with the atoms of the insulation material resulting in accelerated breakdown of the insulation material. If the insulator is used in a electrical device such as a transformer, breakdown of the insulator could cause failure of the transformer. Failure of a transformer used by an electric generating power company could result in the explosion thereof causing injury to personnel, destruction of valuable property and interruption of electric power service to consumers. For these reasons, devices which can detect and monitor partial discharge in electrical devices are greatly needed.
Many attempts have been made at developing a device or system for measuring, monitoring and/or detecting partial discharge. One of the early innovators of improved partial discharge measuring devices was Vogel. U.S. Pat. No. 2,996,664 discloses a device, called a Corona Detector, to which he contributed. The detector seen therein utilizes an oscilloscope to directly display the charge, in coulombs, of a partial discharge pulse emanating from a piece of electrical equipment to be tested. The Vogel device employs a tuned transformer whose secondary winding produces a series of oscillations that directly indicates the charge of the partial discharge pulse in response to the primary winding being excited by the pulse. Unfortunately, the Vogel device does nothing more than detect partial discharge pulses and display a wave form on an oscilloscope. Nothing in Vogel suggests controlling the piece of equipment being tested nor providing a warning signal that the insulation in the electrical device is reaching a critical failure state. Further, the Vogel device requires that the user understand the operation of an oscilloscope, a device which renders readings which are very subjective. Ii is common for the results displayed on a oscilloscope to be interpreted differently by two or more users.
Many other attempts have been made to develop devices and methods for detecting partial discharge occurring in electrical systems. Some devices have employed antennas for receiving electromagnetic radiation from power transmissions lines and other devices where partial discharge may occur. Two such devices are shown in U.S. Pat. Nos. 4,775,839 to Kosina et al. and 5,726,576 to Miyata et al. Unfortunately, the use of an antenna for receiving signals relating to partial discharge has many disadvantages. One such disadvantage is the possibility of receiving unrelated electromagnetic radiation signals thereby producing a false reading for the actual device or system to be tested. Elaborate filtering circuits are needed to eliminate these false reads thereby raising the cost and technical sophistication of the partial discharge detecting device. Even with filtration, due to a lack of a controlled test environment (i.e., shielding or other means of containment), random disturbances, known to exist on multiple levels within the electromagnetic spectrum in the ambient air, can contribute to a corrupted test result. Examples of random disturbances include, solar and microwave radiation, beat frequency oscillations, lightening, RF from fluorescent lighting and other naturally and man made occurring phenomenon. Further, if the electrical system to be tested is a shielded power transformer, wherein multiple transformers are located within close proximity of one another (i.e., a power sub-station), it would be difficult to isolate and test a single transformer in the sub-station through the use of a device receiving a signal by means of an antenna. Even presuming proper isolation of a particular signal emanating from a particular piece of equipment, the reception of the signal utilizing an antennae is still extremely "position sensitive." For instance, since RF and acoustic signals follow the inverse square law, an operator would have great difficultly ascertaining whether the received signal has been attenuated; there is essentially no reference point. Further, the received signal could have been manipulated and/or distorted due to various wave propagation anomalies such as reflection, diffraction and refraction.
Yet other attempts at detecting partial discharge have resulted in the development of devices that apply a high frequency AC voltage test signal to the electrical system to be tested in order to determine whether any partial discharge will occur. Such a device can be seen in U.S. Pat. No. 5,365,177 to Hamp, III et al. Inherent disadvantages exist with this type of device, such as, for example, the necessity of providing the AC test voltage. One of the great needs for partial discharge detection devices is that systems in the field, such as power transformers, need to be tested for partial discharge. The operator testing such a transformer is hampered by the need to apply an AC test voltage in the field. Further, in utilizing the Hamp III device, the system to be tested must be removed from operation, thereby preventing a system test under normal operating and load conditions.
Yet even further attempts at improving partial discharge detection devices can be seen in U.S. Pat. Nos. 4,897,607 to Grunewald et al., 4,967,158 to Gonzalez, and 5,506,511 to Nilsson et al. These devices employ a method of detecting partial discharge through the measurement and analyzation of high frequency sound waves attributed to partial discharge through the use of transducers, microphones and other sound wave detecting devices. Unfortunately, inherent disadvantages in the use of such devices exist. For example, naturally occurring and man-made acoustic phenomenon exist in all frequencies and incident and co-incident phase modes in ambient air. Such phenomenon is known to be detected by transducers, microphones and the like. It is therefore necessary to employ filtration circuitry in an attempt to remove the undesired random signals from the actual signal to be analyzed. Without filtration, it would be difficult to determine that the reading produced by the detection device is actually that of a partial discharge signal. Further, in the case that the electrical system to be tested is a transformer, the sound wave receiving devices of these prior art references are susceptible to vibrations of the transformer tank walls. In particular, as an acoustic signal propagates from the partial discharge point, it travels through the insulating medium and eventually strikes the tank wall. Accordingly, if a microphone is attached to the tank wall, the signal that the microphone receives may be that of the signal traveling through the steel wall, in that sound waves travel quicker through a solid material than through a liquid or gas. Further, all of these prior art devices require that the system be analyzed in a "pure" test environment. In other words, the system needs to be taken "off-line." Additionally, pure test environments should include the use of copper shielded rooms or anechoic chambers to ensure that no random disturbances can effect the test results. These type of testing rooms are expensive to build and maintain. In regards to instrument transformers, as used by utility companies, taking them off-line can have detrimental economic consequences, since instrument transformers are used for consumer billing purposes. Still further deficiencies in these prior art devices are that the Nilsson device will not work in a dry-type transformer. And, even though the Gonzalez device incorporates alarm circuitry for alerting that a fault is about to occur, nothing disclosed therein teaches or suggests that the alarm circuitry should work in tandem with switching and/or relaying circuitry which could take the piece of equipment off-line. Further, nothing in Gonzalez suggests or teaches remote monitoring and/or alarming.
As discussed above, many disadvantages exist within the prior art. Most prior art devices require that the electrical system to be tested be taken off-line for the purpose of the test. Further, many of the prior art devices lack portability. Still further, most prior art devices employ detection technology that is susceptible to interference from random electromagnetic radiation and corrupted signals.
An improved device is needed which overcomes all of the deficiencies seen in the prior art. In particular, the device should be unobtrusive (i.e., passive in nature) such that the electrical system to be tested can remain "on-line" during testing thereof. But, the device should ensure that no feedback is introduced into the system if the system is to remain "on-line" during the test. Further, the improved device should be portable, thereby permitting a technician to take partial discharge readings in the field, regardless of the remoteness of the location. Still further, the device should be designed with detection technology that is more impervious to interference from naturally occurring and man-made electrical phenomenon without the need of sophisticated filtering circuitry or special testing environments (copper shielded room and/or anechoic chamber). Yet still further, the device should incorporate a means for alarming that a fault is possible as well as a means for controlling (i.e., shutting down) the device being tested/monitored in response to the alarm. Yet still even further, the device should be inexpensive and easy to manufacture.